This invention relates to making of high strength thin cast strip, and the method for making such cast strip by a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated, internally cooled casting rolls so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal is poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip to form a casting pool of molten metal, supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
In the past, high-strength low-carbon thin strip with yield strengths of 413 MPa (60 ksi) and higher, in strip thicknesses less than 3.0 mm, have been made by recovery annealing of cold rolled strip. Cold rolling was required to produce the desired thickness. The cold roll strip was then recovery annealed to improve the ductility without significantly reducing the strength. However, the final ductility of the resulting strip still was relatively low and the strip would not achieve total elongation levels over 6%, which is required for structural steels by some building codes for structural components. Such recovery annealed cold rolled, low-carbon steel was generally suitable only for simple forming operations, e.g., roll forming and bending. To produce this steel strip with higher ductility was not technically feasible in these final strip thicknesses using the cold rolled and recovery annealed manufacturing route.
In the past, high strength, steel has been made by microalloying with elements such as niobium, vanadium, titanium or molybdenum, and hot rolling to achieve the desired thickness and strength level. Such microalloying required expensive and high levels of niobium, vanadium, titanium or molybdenum and resulted in formation of a bainite-ferrite microstructure typically with 10 to 20% bainite. See U.S. Pat. No. 6,488,790. Alternately, the microstructure could be ferrite with 10-20% pearlite. Hot rolling the strip resulted in the partial precipitation of these alloying elements. As a result, relatively high alloying levels of the Nb, V, Ti or Mo elements were required to provide enough age hardening of the predominately ferritic transformed microstructure to achieve the required strength levels. These high microalloying levels significantly raised the hot rolling loads needed and restricted the thickness range of the hot rolled strip that could be economically and practically produced. Such alloyed high strength strip could be directly used for galvanizing after pickling for the thicker end of the product range greater than 3 mm in thickness.
However, making of high strength, steel strip less than 3 mm in thickness with additions of Nb, V, Ti or Mo to the base steel chemistry was very difficult, particularly for wide strip due to the high rolling loads, and not always commercially feasible. In the past, large additions of these elements were needed for strengthening the steel, and in addition, caused reductions in elongation properties of the steel. High strength microalloyed hot rolled strips in the past were relatively inefficient in providing strength, relatively expensive, and often required compensating additions of other alloying elements.
Additionally, cold rolling was generally required for lower thicknesses of strip; however, the high strength of the hot rolled strip made such cold rolling difficult because of the high cold roll loadings required to reduce the thickness of the strip. These high alloying levels also considerably raised the recrystallization annealing temperature needed, requiring expensive to build and operate annealing lines capable of achieving the high annealing temperature needed for full recrystallization annealing of the cold rolled strip.
In short, the application of previously known microalloying practices with Nb, V, Ti or Mo elements to produce high strength thin strip could not be commercially produced economically because of the high alloying costs, relative inefficiency of element additions, difficulties with high rolling loads in hot rolling and cold rolling, and the high recrystallization annealing temperatures required.
A steel product is disclosed comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and niobium between about 0.01% and about 0.20% and having a majority of the microstructure comprised of bainite and acicular ferrite and having more than 70% niobium in solid solution. Alternately, the niobium may be less than 0.1%. The steel product may further comprise at least one element selected from the group consisting of molybdenum between about 0.05% and about 0.50%, vanadium between about 0.01% and about 0.20%, and a mixture thereof.
The steel product may have a yield strength of at least 340 MPa, and may have a tensile strength of at least 410 MPa. The steel product may have a yield strength of at least 485 MPa and a tensile strength of at least of at least 520 MPa. The steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
The steel product may be a thin cast steel strip. Optionally, the thin cast steel strip may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers
The thin cast steel strip may have a thickness of less than 2.5 mm. Alternately, the thin cast steel strip may have a thickness of less than 2.0 mm. In yet another alternative, the thin cast steel strip may have a thickness in the range from about 0.5 mm to about 2 mm.
The hot rolled steel product of less than 3 millimeters thickness is also disclosed comprised, by weight, of less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05% and 0.50% silicon, less than 0.01% aluminum, and niobium between about 0.01% and about 0.20%, and have a majority of the microstructure comprised of bainite and acicular ferrite and capable of providing a yield strength of at least 410 MPa with a reduction of between 20% and 40%. The steel product may have a yield strength of at least 485 MPa and a tensile strength of at least of at least 520 MPa. Alternately, the niobium may be less than 0.1%.
Optionally, the hot rolled steel product may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers.
The hot rolled steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%. The hot rolled steel product may have a thickness of less than 2.5 mm. Alternately, the hot rolled steel product may have a thickness of less than 2.0 mm. In yet another alternative, the hot rolled steel product may have a thickness in the range from about 0.5 mm to about 2 mm.
Also disclosed is a coiled steel product comprised, by weight, of less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and at least one element selected from the group consisting of niobium between about 0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%, and a mixture thereof, and having more than 70% niobium and/or vanadium in solid solution after coiling and cooling. Alternately, the niobium may be less than 0.1%.
Optionally, the coiled steel product may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers.
The coiled steel product may have a yield strength of at least 340 MPa, and may have a tensile strength of at least 410 MPa. The coiled steel product has a thickness of less than 3.0 mm. The steel product may have a yield strength of at least 485 MPa and a tensile strength of at least of at least 520 MPa.
Alternately, the coiled steel product has a thickness of less than 2.5 mm. Alternately, the coiled steel product may have a thickness of less than 2.0 mm. In yet another alternative, the coiled steel product may have a thickness in the range from about 0.5 mm to about 2 mm. The coiled steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
An age hardened steel product is also disclosed comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, at least one element from the group consisting of niobium between about 0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%, and a mixture thereof, and having a majority of the microstructure comprised of bainite and acicular ferrite and having an increase in elongation and an increase in yield strength after age hardening. Alternately, the niobium may be less than 0.1%.
The age hardened steel product may comprise, in addition, fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers.
The steel product may have a yield strength of at least 340 MPa, or at least 380 MPa, or at least 410 MPa, or at least 450 MPa, or at least 500 MPa, or at least 550 MPa, or at least 600 MPa, or at least 650 MPa, as desired. The steel product may have a tensile strength of at least 410 MPa, or at least 450 MPa, or at least 500 MPa, or at least 550 MPa, or at least 600 MPa, or at least 650 MPa, or at least 700 MPa, as desired. The age hardened steel product has a thickness of less than 3.0 mm. Alternately, the age hardened steel product has a thickness of less than 2.5 mm. Alternately, the age hardened steel product may have a thickness of less than 2.0 mm. In yet another alternative, the age hardened steel product may have a thickness in the range from about 0.5 mm to about 2 mm. The age hardened steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
A steel product comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and at least one element selected from the group consisting of niobium between about 0.01% and about 0.20% and vanadium between about 0.01% and about 0.20%, and having a majority of the microstructure comprised of bainite and acicular ferrite and comprising fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. Alternately, the niobium may be less than 0.1%. Optionally, the steel product may comprise molybdenum between about 0.05% and 0.50%.
The steel product may have a yield strength of at least 340 MPa, and may have a tensile strength of at least 410 MPa. The steel product may have a yield strength of at least 485 MPa and a tensile strength of at least of at least 520 MPa. The steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
An age hardened steel product comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and niobium between about 0.01% and about 0.20%, and having a majority of the microstructure comprised of bainite and acicular ferrite and having niobium carbonitride particles with an average particle size of less than 10 nanometers. Carbonitride particles, in the present specification and appended claims, includes carbides, nitrides, carbonitrides, and combinations thereof. Alternately, the niobium may be less than 0.1%.
The age hardened steel product may have substantially no niobium carbonitride particles greater than 50 nanometers. The age hardened steel product may have a yield strength of at least 340 MPa, and may have a tensile strength of at least 410 MPa. The age hardened steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
A method is disclosed for preparing coiled thin cast steel strip comprising the steps of:                assembling internally a cooled roll caster having laterally positioned casting rolls forming a nip between them, and forming a casting pool of molten steel supported on the casting rolls above the nip and confined adjacent the ends of the casting rolls by side dams,        counter rotating the casting rolls to solidify metal shells on the casting rolls as the casting rolls move through the casting pool, and        forming from the metal shells downwardly through the nip between the casting rolls a steel strip, and        cooling the steel strip at a rate of at least 10° C. per second to provide a composition comprising by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and at least one element selected from the group consisting of niobium between about 0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%, and a mixture thereof, and having a majority of the microstructure comprised of bainite and acicular ferrite and having more than 70% niobium and/or vanadium in solid solution.        
The method may provide in the steel strip as coiled fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. Further, the method may comprise the steps of hot rolling the steel strip, and coiling the hot rolled steel strip at a temperature between about 450 and 700° C. Alternately, the coiling of the hot rolled steel strip may be at a temperature less than 650° C.
The method may further comprise the step of age hardening the steel strip to increase the tensile strength at a temperature of at least 550° C. Alternately, the age hardening may occur at a temperature between 625° C. and 800° C. In yet another alternate, the age hardening may occur at a temperature between 650° C. and 750° C.
Also disclosed is a method of preparing a thin cast steel strip comprised the steps of:                assembling internally a cooled roll caster having laterally positioned casting rolls forming a nip between them, and forming a casting pool of molten steel supported on the casting rolls above the nip and confined adjacent the ends of the casting rolls by side dams,        counter rotating the casting rolls to solidify metal shells on the casting rolls as the casting rolls move through the casting pool, and        forming steel strip from the metal shells cast downwardly through the nip between the casting rolls, and        cooling the steel strip at a rate of at least 10° C. per second to provide a composition comprising by weight, less than 0.25% carbon, less than 0.01% aluminum, and at least one element from the group consisting of niobium between about 0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%, and a mixture thereof, and having a majority of the microstructure comprised of bainite and acicular ferrite and having more than 70% niobium and/or vanadium in solid solution,        age hardening the steel strip at a temperature between 625° C. and 800° C.        
The method may further comprise the step of age hardening the steel strip to increase the tensile strength. Alternately, the age hardening may occur at a temperature between 650° C. and 750° C.
The method may provide the age hardened steel strip having niobium carbonitride particles with an average particle size of less than 10 nanometers. Alternately, the age hardened steel strip has substantially no niobium carbonitride particles greater than 50 nanometers.
The method may provide in the steel strip as coiled fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. Further, the method may comprise the steps of hot rolling the steel strip, and coiling the hot rolled steel strip at a temperature less than 700° C. Alternately, the coiling of the hot rolled steel strip may be at a temperature less than 650° C.
The method of preparing a thin cast steel strip may comprise the steps of:                assembling internally a cooled roll caster having laterally positioned casting rolls forming a nip between them, and forming a casting pool of molten steel supported on the casting rolls above the nip and confined adjacent the ends of the casting rolls by side dams,        counter rotating the casting rolls to solidify metal shells on the casting rolls as the casting rolls move through the casting pool; and        forming from the metal shells downwardly through the nip between the casting rolls a steel strip; and        cooling the steel strip at a rate of at least 10° C. per second to provide a composition comprising by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and at least one element from the group consisting of niobium between about 0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%, and a mixture thereof, and having a majority of the microstructure comprised of bainite and acicular ferrite,        age hardening the steel strip at a temperature between 625° C. and 800° C. and having an increase in elongation and an increase in yield strength after age hardening.        
The method may provide in the steel strip as coiled fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. Further, the method may provide the age hardened steel strip having niobium carbonitride particles with an average particle size of less than 10 nanometers. Alternately, the age hardened steel strip has substantially no niobium carbonitride particles greater than 50 nanometers.
The method may comprise the steps of hot rolling the steel strip, and coiling the hot rolled steel strip at a temperature less than 750° C. Alternately, the coiling of the hot rolled steel strip may be at a temperature less than 700° C.